The present invention generally pertains to cone beam computed tomography (CT) imaging apparatus and method. More particularly, the invention pertains to such apparatus and method that acquires and processes cone beam projection data acquired along a trajectory comprising a circular or other primary scan path (i.e., orbit) supplemented by a helical or other supplementary scan path.
Cone beam CT imaging has developed as an important technique in constructing a three-dimensional CT image. According to such technique, an X-ray source irradiates the object with conical shaped X-rays while traversing a prescribed scan path or trajectory, to project an image of the object, in the form of cone beam X-ray data, onto an array of two-dimensional detector elements. The detector elements acquire or receive the projected cone beam data, which is then processed to provide the reconstructed image of the object.
Scan path is an essential consideration in cone beam imaging. Different scan paths represent different data measurement procedures and call for different data processing algorithms (reconstruction algorithms) to produce the reconstructed images. Developing accurate, efficient and robust reconstruction algorithms for the scan paths of practical interest has been the focus of many research groups. As a prerequisite for high fidelity (exact) reconstruction, the scan path employed should provide sufficient cone beam data measurements.
The reconstruction algorithm for generating a primary part of the reconstructed function from circular path cone beam CT was given by Feldkamp et al, "Practical Cone-beam Algorithm", J. Opt. Soc. Am., pp. 612-619 (1984). The algorithm for generating the entire portion of the reconstructed function that can be derived from circular path cone beam CT was recently given by U.S. Pat. No. 5,400,255, issued Mar. 21, 1995, to Hui Hu, the inventor herein. However, it is well known that the circular scan path is likely to provide insufficient cone beam data and may generate erroneous results.
Various scanning geometries (paths) have been developed to ensure that sufficient data is acquired. In one such geometry, the scan path comprises a circular path in combination with a linear path, which is orthogonal to the plane containing the circular path. Various algorithms are currently available for use in processing cone beam data acquired by scanning along a combined circle-and-line path and constructing an image therefrom. However, some of such algorithms, such as set forth in an article by H. Kudo and T. Saito, entitled, "Derivation and implementation of a cone-beam reconstruction algorithm for non-planar orbits", IEEE Trans. Med. Imag. vol. 13 pp. 196-211 (1994) require excessive data processing resources. Other of such algorithms, such as set forth in an article by G Zheng and G. Gullberg entitled, "A cone beam tomography algorithm for orthogonal circle-and-line orbit", Phys. Med. Biol., vol. 37(4) pp. 563-577 (1992) and in U.S. Pat. No. 5,170,439 have been found to be inaccurate.
More recently, an reconstruction algorithm has been developed by the inventor for generating, from the linear scanned data, a portion of the reconstructed function supplementary to the primary portion derivable from the circular scan. Thus, this supplementary portion is then additively combined with the primary portion which is derived from the circular scan in accordance to U.S. Pat. No. 5,400,255 to provide a complete reconstruction of the function of the object. While this technique has provided significant benefits in terms of the reconstruction accuracy, it has been found that a substantial amount of processing effort is still required, in order to derive the linear scan portion of the reconstructed function. It would be desirable to significantly reduce the data processing load by improving the efficiency of the technique.
The circle-and-line scan path is of great practical interest, since it can be readily implemented by rotating the scanner or the object around a circle in the circular scan and by translating the object along the axis of rotation in the linear scan. However, since no rotation is allowed during the linear scan, the time it takes to completely stop the rotation before the linear scan and to reestablish the rotation for the sequential circular scan after the linear scan is too long for some applications, especially when using the circle-and-line scan repeatedly. To eliminate he lengthy switching time and therefore increase the overall data acquisition speed, the present invention proposes a new scan path, i.e., the circle-and-helix scan path. It would be desirable to develop a reconstruction algorithm for this scan path.
For most applications in medicine and some applications in industry, the longitudinal extent of the object to be imaged exceeds the length which can be scanned by the scanner in one scan. Such an object is referred to as a longitudinally-unbounded object. One practical consideration in cone beam CT system development is how to image the longitudinally-unbounded object when only a portion of it is of interest or can be imaged in one scan due to the limited detector extent. Most of the methods developed cannot meet this challenge. It would be desirable to be able to develop a strategy for exact reconstruction of the longitudinally-unbounded object through a series of regional scans and reconstructions.